This invention relates to a polyester container and particularly to such a container having an improved base configuration.
Polyester containers have been replacing metal and glass containers with increasing frequency. The popularity of these products stems in part to improvements in resin composition, manufacturing processes, and container designs. Typical polyester containers such as those made from polyethylene terephthalate (PET) material are formed in a process in which an elongated tubular preform made by injection molding or other processes is heated and placed into a blow molding cavity. A pressure differential is applied which causes it to expand to conform to the inside surface of the mold cavity, thus providing a semi-rigid thin-walled container. Since the container is exposed to various pressures and forces during processing and use as will better be explained below, it must be designed to respond to such physical influences while maintaining a designed configuration. Random or asymmetrical buckling or deformation of the container would produce an esthetically and commercially unacceptable product.
Containers must be designed to be stable when set on a horizontal surface. In the past, many polyester containers were designed to have a rounded bottom which required a separate base component which was glued to the container to provide a flat support plane. More recent polyester container designs, however, are integral structures having a bottom which forms an outer support ring with a central outwardly concave depressed center, often referred to as a "champagne bottom". In addition to the requirements of maintaining a desired configuration, there is a further need to design the container to minimize the quantity of material needed to form it. In the past, polyester containers were designed with a reinforced base having ribs or webs of increased thickness of polyester material which tended to increase the mass of raw material needed to form the product.
During the production cycle of a blow molded polyester container, the preform is typically axially stretched and inflated to impart radial elongation to the material. In the art, such forming is known as biaxial elongation. Such elongation imposes retractive stresses in the material which, if not relaxed or physically restrained, tend to cause the article to shrink and deform in certain conditions in the directions of elongation. The influence of such unrelaxed retractive stresses is particularly significant during certain phases of the production cycle of the container. Immediately after demolding of the container, the elevated temperature of the material causes it to be less rigid than the final product. Accordingly, such unrelaxed retractive stresses tend to have more influence during this phase of the production cycle.
In the past, most polyester containers were used to contain liquids that are initially dispensed into the container at room temperature or chilled. Presently, however, there is more interest in using polyester containers for so-called "hot-fill" applications where the beverage or product is dispensed in the container initially at an elevated temperature and is then immediately sealed. Hot-fill applications impose additional mechanical stress inputs to the container structure. Immediately after the hot liquid is dispensed into the container, its temperature decreases the rigidity of the polyester material, thus making it more subject to the unrelaxed retractive stresses mentioned previously. The container must sustain internal pressure changes while maintaining its configuration. For example, as the hot-filled liquid cools, it shrinks in volume which has the effect of producing a negative pressure in the container. In use, the container must also be resistant to deformation when being handled or dropped which causes sudden increases in internal pressure.
In accordance with this invention, a polyester container is provided having an improved design base structure which provides structural rigidity and resistance against random deformation and shrinkage in response to the previously mentioned mechanical and thermal stresses.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.